LEDs are typically formed as dies having an anode terminal and a cathode terminal. An LED die is typically mounted on a larger substrate for heat dissipation and packaging. The substrate may contain additional circuitry, such as a passive electrostatic discharge device. The LED die and optional substrate are then typically packaged, where the package has robust anode and cathode leads for being soldered to a printed circuit board (PCB).
LEDs may be controlled by an external current source to achieve a desired brightness. The current source may be a MOSFET or a bipolar transistor formed in a separate die. The current source and LED are typically connected together by wires or a PCB.
Providing the current source separate from the LED die requires extra space and interconnections, adding cost. Other disadvantages exist, including the possibility of mismatching components.
The Applicant had previously invented a very compact LED module with an integrated driver transistor, disclosed in U.S. Pat. No. 9,577,007, incorporated herein by reference. In that patent, single-chip LED modules are described with an integrated, lateral-channel PMOS or NMOS transistor. The LED and MOSFET are connected in series on the die to form a 3-terminal device. The LED is turned on by controlling the gate of the MOSFET.
The LED and MOSFET are formed over the same substrate, such as a silicon substrate, an SiC substrate, a GaN substrate, or a sapphire substrate. In the case of a sapphire substrate, the sapphire substrate is removed so that the module can conduct current vertically. Although the MOSFET channels are lateral (using a top lateral gate), the current flow between the top LED electrode and the die's bottom electrode is vertical by using a vertical conductive path between the lateral channel and the bottom electrode. The vertical conductive path may be the doped substrate or a laser-drilled hole filled with a conductive material.
Since the channels are lateral (on the top surface of the die), the surface area of the module is relatively large, adding expense and size. This configuration may prevent the LED modules being used as compact pixels in a display. Further, the long vertical conduction path through the substrate material results in power loss. Laser-drilling holes through the substrate and filling the holes with a conductor add considerable expense.
What is needed is an improved LED module having a MOSFET and LED formed on the same substrate and connected in series, where current is conducted vertically between a bottom electrode and a top electrode, and where the LED module does not have the drawbacks discussed above.